code: def test_zero_images():
      assert np.isnan(sobel(np.zeros((16, 16)))).sum() == 0
      assert sobel(np.zeros((16, 16)), just_filter=True).sum() == 0
  
file_path: mahotas/mahotas/tests/test_edge.py
imports: from mahotas.edge import sobel
  import pytest
  import mahotas
  import numpy
label_desc: Use image node_id 4 for calling a global function with example usage: test_zero_images() without return types